Various methods of encoding video images are known in the art. Currently preferred encoding methods are based on Discrete Cosine Transformation (DCT) by which blocks of pixels, or blocks of differences between actual and predicted pixels, are transformed into coefficients representing spectral components of the video image. The DCT-coefficients of each block are scanned in a predetermined order corresponding to their significance. The first coefficient is the most significant one. This coefficient generally represents the DC level of the block and is further referred to as the DC coefficient. The others are AC coefficients. After quantization, the coefficients may be applied to a Variable Length Coder (VLC).
When subjected to variable length coding, each block of pixels is conveyed as a sequence of codewords of variable length. In order to avoid a total loss of synchronization when a transmission error occurs, a special sync code is transmitted regularly to indicate the start of a group of blocks. A transmission error now only affects the recovery of codewords being transmitted until the next sync code.
Despite the sync code, a problem of the transmission format mentioned above remains its lack of robustness against transmission errors. A single transmission error may still result in erroneous decoding of VLC-codewords up to the detection of a subsequent sync code. Consequently, a transmission error causes the loss of a number of DCT-blocks and thus affects a significant part of the video image.
Similar problems arise in video recorders. When digitally encoded video images are played back at a higher than recording speed, only part of the data on each track is read out. Substantial parts of the data for an image are now not received at all. An approach to this problem is given in Patent Application EP-A 0 509 594. In accordance with the method and apparatus disclosed in that application, the image is recorded on the recording medium in such a way that bursts of data read from the tape at the highest speeds each constitute a macro-segment defining an area of the image. Further, a different segment, i.e. a different area of the image, is read out during each of a plurality of sequential frames. These segments are stored until all pans of an image have been assembled. This method and apparatus do not compromise picture quality during playback at the recording speed. The operation in search modes is also good when images with relatively low motion content are concerned. For images with higher motion content some improvement is desirable.